ER Diagram Online

Example ER diagram code

This is the Mermaid source loaded in the editor above. Copy it into a Markdown file inside a ```mermaid block and GitHub, GitLab, Notion or Obsidian will render it.

erDiagram
    CUSTOMER ||--o{ ORDER : places
    ORDER ||--|{ ORDER_ITEM : contains
    PRODUCT ||--o{ ORDER_ITEM : "appears in"
    CUSTOMER {
        int id PK
        string email UK
        string name
    }
    ORDER {
        int id PK
        int customer_id FK
        date created_at
        string status
    }
    ORDER_ITEM {
        int order_id FK
        int product_id FK
        int quantity
    }
    PRODUCT {
        int id PK
        string sku UK
        decimal price
    }

ER diagram syntax cheat sheet

How to make a Mermaid ER diagram

1. 1Start the code with erDiagram.
2. 2Add relationships such as CUSTOMER ||--o{ ORDER : places.
3. 3List each entity's attributes with type and name in braces.
4. 4Mark keys with PK, FK or UK.
5. 5Export as SVG or PNG, or copy a share link.

Crow's foot cardinality

Each side of a relationship has two characters: |o means zero or one, || exactly one, }o zero or more and }| one or more. They are mirrored on the right side, so CUSTOMER ||--o{ ORDER reads "one customer has zero or more orders".

A relationship needs a label after the colon. Use a verb such as places or contains so the diagram reads as a sentence.

Attributes and keys

Add columns in braces after the entity name, one per line as type then name, such as int id PK and string email UK.

PK marks a primary key, FK a foreign key and UK a unique key. You can add a quoted comment after the key to note constraints.

Identifying vs non-identifying

Two dashes (--) draw a solid line for an identifying relationship, where the child cannot exist without the parent. Two dots (..) draw a dashed line for a non-identifying one.

Model one bounded area at a time; a diagram of 10 to 15 entities stays readable.